The preferred embodiment of the present invention is used to intelligently manage junction temperature in a actuator controller that positions the read/write heads (or transducers) in a magnetic disc drive. For that reason, the background of this invention will be described with respect to temperature monitoring such controllers. However, the present invention may be used to monitor and control junction temperature in other systems as well.
As shown by FIG. 1, a conventional magnetic disc storage system 2 includes one or more magnetic storage platters or discs 4, 6 that are rotated by a spindle motor 8. Discs 4, 6 have respective upper and lower surfaces 4U, 4L, 6U, 6L upon which data may be magnetically written or read. Projecting arms of an actuator assembly 10 carry read/write heads (hereafter "heads" or "transducers") 12U, 12L, 14U, 14L that respectively read and/or write data from disc surfaces 4U, 4L, 6U, 6L. Actuator assembly 10 moves all heads radially under command of a positioning servo controller system 16. A flexible multiconductor cable (not shown) couples the actuator assembly 10 to the servo controller system 16. Of course, disc storage assembly 2 is contained in a suitably sealed protective housing (not shown). Controller system 16 typically includes at least one integrated circuit ("IC") 30, whose junction temperature must not be permitted to exceed a threshold level for too long a period of time.
The various surfaces of discs 4, 6 are commonly formatted into concentric tracks, T1, T2, T3, etc., portions of which are defined as pie-shaped wedges or sectors, e.g., S1, S2, etc. As such, the various disc storage locations may be defined by disc number (e.g., disc 4), disc surface (e.g., 4U), track (or cylinder) number (e.g., T3) and sector number (e.g., sector S1).
Disc system 2 is coupled via a suitable interface to a host computer (not shown). In response to commands issued by the host computer user, or by a program under execution by the host computer, an appropriate disc drive interface command is issued. For example, one such command may require the servo controller 16 to seek data or a storage location on one of the surfaces of a disc, e.g., disc 4, surface 4U, track T3, sector S2.
In response to the command, servo controller system 16 actuates assembly 10 in a controlled fashion. Assembly 10 moves heads 12U, 12L, 14U, 14L in unison over the disc surfaces until the heads are positioned over the desired target track, T3 in the example at hand. Since all heads on the carriage move together, system 2 includes control circuitry to select the proper read/write head to perform the desired data transfer function, head 12U in the present example. Typically system 2 accesses data from the target location within perhaps ten milliseconds.
Some storage systems provide a dedicated disc surface upon which positional information is permanently prerecorded or embedded for use by servo system 16, for example surface 4U. By demodulating the pre-recorded servo information from this disc surface, the appropriate heads permit a positional error signal ("PES") to be derived. The PES corresponds to head positional error from the intended track center and can permit derivation of track crossing information.
Modern magnetic storage systems tend to use smaller discs whereon a higher density of data is to be reliably recorded and retrieved. Thus, it is not always feasible to dedicate a disc surface for storage of positional data. In these non-dedicated sector servo systems, servo information is recorded interspersed with user data. Using the embedded servo data, the read heads sense the sector over which the heads are positioned. Servo controller system 16 then re-positions actuator assembly 10 until the heads are positioned over the desired target track, and sector thereon.
In either type system, positional data are sampled by a read/write head, whereupon servo system 16 outputs positional information including PES. Generally this output positional information drives actuator assembly 10, which repositions the read/write heads (e.g., 12U, 12L) as required. More specifically, a stepper motor or a voice coil mechanism with actuator assembly 10 repositions the heads.
As shown by FIG. 1, servo system 16 generally includes an actuator controller IC 30 that receives input commands from the host computer (not shown). In response, actuator control IC 30 outputs control signals to the actuator assembly 10, which repositions itself accordingly. Typically the actuator controller IC 30 receives DC operating voltage from an external power source via a power series pass unit 32 that is generally in close physical proximity.
FIG. 2 depicts system-level operation of head positioning, and demonstrates the role of the actuator controller IC, which is shown generally as 30. Output from IC 30 is a drive current signal Io that drives the actuator and heads, shown collectively as 10. A read/write head preamplifier 34 detects magnetically encoded data on the adjacent disc surface and couples such data to a demodulator/pulse detector unit 36.
Unit 36 also receives reset and latch signals from a so-called glue chip 28, and outputs digital encoded read data and a positional error signal ("PES"). Glue chip modules are known in the art, and provide control functions that include power-on reset, chip select logic, write fault logic, spindle speed control, dual PWM digital-to-analog converted outputs, as well as embedded servo decode functions.
In FIG. 2, glue chip 28 communicates with a microcontroller unit 22 associated with the servo system, and includes a pulse width modulation ("PWM") digital/analog converter unit 44 and an associated servo data logic unit. Microcontroller unit 22 typically includes an analog/digital converter 24, a microprocessor 26, and various memories 37, 38, and 42. Microprocessor 22 receives the PES, which is digitized for further signal processing.
Upon detecting an encoded servo mark in the data provided by the demodulator pulse detector 36, glue chip 28 generates signals that sequentially reset (e.g., discharge) and then charge capacitors within demodulator 36. These capacitors then latch and hold the corresponding average burst signals, from which the PES information is derived. The glue chip 28 also generates servo interrupt signals for microcontroller unit 22, and sector mark data containing guardband and index information.
As further shown by FIG. 2, PWM data from glue chip 28 is coupled to unit 30, where the data are low-pass filtered by filter 46 and amplified by amplifier 48. The resultant output current Io is coupled to drive the actuator and heads, collectively 10.
It is important that controller IC 30 not overheat while positioning the read/write heads. Such overheating can result in improper output positioning signals, and can also result in permanent damage to IC 30 itself. Understandably, absent a viable controller IC 30, the associated hard disc drive system 2 is useless.
With further reference to FIG. 1, it is known to monitor junction temperature in the power series pass unit 32 that provides DC operating voltage to the controller IC 30. Typically the power series pass unit 32 temperature is monitored by measuring DC voltage across a forward biased emitter-base diode junction in the output stage of unit 32. As is well known in the art, this DC voltage will change as a function of junction temperature, and the voltage change across the diode junction provides a measure of temperature.
In practice, overheating in power supply unit 32 is considered as an indication of imminent overheating within the actuator controller IC 32. Thus, as power unit 32 overheat, it shuts down, which causes a catastrophic shutdown of the actuator controller IC 30. In FIG. 1, this shutdown mechanism is shown generically as a switch S1. However, it is understood that logic circuitry within unit 32 simply disables the operating output voltage provided to IC 30. Because it cannot function without DC operating voltage, IC 30 also shutsdown, and thus does not function in what may be an overheated environment. While the above described procedure can protect actuator controller IC 30 against possible overheating, the resultant IC 30 shutdown is catastrophic. Data in the process of being written to or read from the associated magnetic disc can be lost or corrupted. Further, prolonged IC 30 shutdown is not transparent to the user of the host computer. If power supply unit 32 remains shutdown for too long, it may be necessary for the user of the computer system associated with the hard disk to reboot the system. Finally, it will be appreciated that catastrophic shutdown of actuator controller IC 30 may occur, even if IC 30 were in no imminent danger of overheating.
Alternative approaches to protecting IC 30 against overheating are also known. Those skilled in the art will appreciate that a controller IC can overheat if forced to output a sequence of pulses having too great a duty cycle. To protect against such overheat, one can enforce a maximum duty cycle limitation on the output pulse signals provided by IC 30. U.S. Pat. No. 4,907,108 to Masuyama (1990) provides a delayed-action actuator motor control circuit that reduces load current output to the actuator itself. More specifically, this reference discloses a logic circuit that introduces an additional time delay whenever an actuator controller IC output seek pulse will be followed too closely in time by an adjacent output seek pulse. In this manner, Masuyama's circuit can prevent overheating by preventing excessive output signal duty cycle.
Unfortunately, Masuyama's circuit will impose a maximum limit on the actuator controller IC duty cycle, even if it is not necessary to do so. For example, if the actuator controller IC is in a cold environment, then a duty cycle higher than what Masuyama's circuit will impose can be safely accommodated. Even in a nominal temperature environment, fabrication tolerance spreads can produce an actuator controller IC that can safely accommodate a greater maximum duty cycle than that for which Masuyama's circuit was designed. Stated differently, Masuyama blindly and passively imposes a maximum duty cycle limit and delays the head positioning process without considering whether actual operating conditions require such delay.
In summary, there is a need for a system that will intelligently prevent a hard disc actuator controller IC from operation under extended overheating operating conditions. Such system should monitor actuator controller IC junction temperature and, upon sensing a possible overheating condition, prevent overheating in a non-catastrophic manner. The actuator controller IC should be prevented from operating in overheating conditions in a manner preferably transparent to a user of the associated hard disc drive system. The present invention discloses such a system.